Per clock cycle, the 6502 could do a lot more compared to the Z-80. For instance, a 6502 can read from memory on every cycle, so it can execute one instruction while fetching the next. A NOP takes four cycles to complete on a Z-80 and only two on a 6502. The Z-80 also has a 4-bit ALU and has to operate on 8 bits on two passes. Finally, another very clever thing with the 6502 is the single byte addressing, which kind of gives it 256 “registers”. Of course, it has a single byte stack pointer, which makes languages like C or PASCAL a terrible match for it. Z-80s on the other hand have much more complex instructions, which make Z-80 code denser, something very important in those days.